This invention generally relates to an improved guide plate of the type used to guide the control rodlets through the guide tubes of a nuclear reactor. The guide plate includes a plurality of vent openings for reducing turbulence in the flow of coolant through the guide plate, which in turn reduces both fretting and frictional engagement between the control rodlets and the guide holes in such guide plates.
The core of a modern nuclear reactor of the type used to generate electrical power generally includes an upper internals assembly disposed over a lower core barrel. The lower core barrel houses an array of fuel assemblies which generate heat as a result of a controlled fission reaction that occurs in the uranium oxide pellets present within the individual fuel rods. Water is constantly circulated from the lower core barrel through the upper internals and out through the outlet ports provided in the walls of an upper core barrel in order to transfer the heat generated by the fuel rod assemblies to heat exchangers which ultimately convert this heat into usable, non-radioactive steam. The upper internals assembly includes an upper core barrel arrangement in tandem with the lower core barrel of the reactor. The ceiling of the upper core barrel is formed from an upper support plate. The peripheral edge of this support plate is seated around the upper edge of the upper core barrel. Both the support plate and the upper core plate which underlies it include a plurality of apertures for conducting the stream of hot, pressurized water exiting the fuel assemblies to the heat exchangers, as well as for conducting what are known in the art as rod cluster control assemblies into the fuel assemblies. The rate of the fission reaction taking place within the fuel assemblies is regulated by means of the rod cluster control assembly. Each of the control rod assemblies is formed of a neutron absorbing substance, such as an alloy of silver, indium and cadmium, that is clad in a tube of stainless steel. The stainless steel tubes (known as "rodlets" in the art) are suspended from a spider-like bracket formed from a plurality of interconnected vanes which in turn interconnect the top end of all of the rodlets. A reciprocable drive rod is connected to the spider-like bracket for either inserting or withdrawing the rodlets either deeper into or farther out of each of the fuel assemblies in order to modulate the amount of heat generated within the fuel assemblies.
As they reciprocate, the rodlets of the rod cluster control assemblies are guided into their respective fuel assemblies by guide tubes. The bottom end of each of these guide tubes is bolted onto the upper core plate which forms the ceiling of the lower core barrel, while the upper portion of each of these guide tubes is laterally supported within an aperture in the upper support plate which forms the ceiling of the upper core barrel of the reactor. While the lower end of each of the guide tubes includes a plurality of guide sheaths having round holes interconnected by slots which guide the rodlets in much the same fashion as a sword is guided into proper alignment within its sheath, the intermediate and upper portion of each guide tube includes a plurality of guide plates for this purpose. Similar to the guide sheaths, each of these guide plates includes a plurality of guide holes interconnected by slots for slidably receiving both the rodlets and the control rod vanes of the rod cluster control assembly. Each of the guide plates further includes a central orifice for conducting coolant through the guide tube. However, while the guide sheath may be 30 to 35 inches (76.2-88.9 cm) long, each of the guide plates is only about one inch (2.54 cm) thick. Hence, the guide plate provides guidance to the rodlets of the rod cluster control assembly with far less frictional engagement than the guide sheath provided at the very ends of each of the guide tubes, which is advantageous in view of the fact that such lowered frictional contact reduces the minimum drop time required to completely insert the rodlet into the fuel assemblies. Such a lowered drop time in turn allows the reactor operators to slow down or stop the nuclear reaction within the fuel assemblies in a shorter period of time in the event of an emergency situation.
While the performance of such prior art guide plates has generally proven to be satisfactory, the applicant has noted a number of areas in which the performance of these guide plates might be improved. For example, over a period of time, it has been observed that the flow of coolant water through the central orifice of these guide plates can induce vibrations in the rodlets which can cause them to rub against the guide holes in the plates and ultimately wear down the stainless steel cladding which forms the outer surface of such rodlets. Additionally, the applicant has observed that the flow of coolant through the central orifice in these guide plates also creates a low pressure area just above the upper surface of the plate which tends to radially pull each of the rodlets into frictional engagement with the guide holes that slidably receive them. Such frictional engagement, in combination with the periodic reciprocable movements of these rodlets during the operation of the reactor, can also cause the stainless steel cladding that forms the outer surface of the rodlets to wear down. The resulting fretting and wear caused by the flow patterns of coolant through such prior art guide plates reduces the life of the rodlets, thereby causing them to be replaced at shorter time intervals than would otherwise be the case. Such rodlet maintenance and replacement is expensive, since it increases reactor down time, and necessitates the opening up of the upper internals assembly of the reactor, which is in itself an expensive operation that exposes maintenance workers to potentially hazardous radiation.
Clearly, there is a need for an improved guide plate that eliminates or at least reduces unwanted fretting and frictional engagement between the rodlets of the rod cluster control assembly and the guide holes in the guide plate so as to minimize wear between these plates in the outer cladding of the rodlets. Ideally, the reduction in fretting and frictional engagement would result in an even faster minimum drop time for the rodlet to be inserted completely into the respective fuel assemblies, which in turn would enhance the safety capabilities of the reactor by allowing the reactor operators to more quickly reduce or stop the nuclear reaction within the fuel assembly in the event such reduction becomes necessary.